1. Field of the Invention
The present invention relates to a wheel condition estimating apparatus, and more particularly to a wheel condition estimating apparatus for estimating a wheel condition from an output response or the like of a wheel resonance system on the basis of a vibration model of the wheel resonance system including a frictional characteristic between a tire and a road surface.
2. Description of the Related Art
In recent years, in accordance with an increase in efforts to improve the safety of automotive vehicles, preventative safety techniques have been researched and developed. An antilock brake systems (ABS), which are a representative safety apparatus, have already been used in many vehicles.
ABS control on the basis of a new principle which pays attention to the resonance phenomenon of the tire has been proposed, and the study thereof continues (Japanese Patent Application No. 7-220920 and the like). This technique is such that a micro vibration having a frequency component equal to a resonance frequency when the tire grips a road is applied to the brake pressure, and the average brake pressure is controlled on the basis of a resonance gain (micro amplitude of wheel speed at the resonance frequency/vibration amplitude of brake pressure) of a tire resonance system at that time.
It is known that the resonance gain is a physical quantity related to the gradient of a coefficient of friction .mu. with respect to a slip ratio S (hereinafter referred to as a .mu. gradient) of a coefficient .mu. of friction in a so-called S-.mu. characteristic (a changing curve of a coefficient of friction .mu. with respect to a Slip ratio S). It is expected that a frictional characteristic, which relates to ease of slipping between a tire and a road surface at a time of braking, can be estimated on the basis of the resonance gain.
However, since the prior art mentioned above presupposes that the resonance frequency of the tire resonance system is known, there is a problem that it cannot immediately respond in the case that the resonance frequency changes due to the tire being replaced or the air pressure in the tire changing. For example, when the air pressure in the tire falls, the resonance frequency at the time when the tire grips the road surface changes to a lower side. Therefore, the value of the resonance gain becomes small, and the accuracy of detecting whether the current state is a state immediately before a peak .mu. value by comparison with a predetermined reference gain deteriorates.
Further, in the above-described prior art, the brake pressure is minutely vibrated at a time of calculating the resonance gain. However, because the micro vibration can be applied only at a time of braking due to the structure of a brake portion, there is a problem that the opportunity to estimate the road surface condition is limited to the time of braking in the case of a vehicle having the brake portion of this type. Still further, there are cases in which micro vibration of the brake pressure is not preferable such as a time of driving, a time of steady running and the like.